[GCC] PR23529 Mangler part of attrbute abi_tag support

Original patch by Stefan Bühler http://reviews.llvm.org/D12834

Difference between original and this one:
- fixed all failing tests
- fixed mangling for global variable outside namespace
- emit ABI tags for guards and local names
- clang-format + other stylistic changes
- significantly reworked patch according to Richard's suggestions

Sema part, committed before http://reviews.llvm.org/D17567

Differential revision: http://reviews.llvm.org/D18035

llvm-svn: 274222